I. Technical Field
The present invention relates to monopulse tracking antennas. More specifically, the present invention relates to a novel and improved lowsidelobe monopulse track antenna utilizing slot radiators arranged in a traveling wave configuration to achieve amplitude comparison monopulse radiation patterns.
II. Background Art
Weapon systems utilizing microwave radar systems generally suffer serious performance degradation in the jamming environment encountered in tactical situations. Since the potential for jamming is high in almost all tactical situations, techniques to improve the electronic countermeasures (ECM) capability of these systems are of great importance.
Presently deployed microwave tracking systems are susceptible to electromagnetic jamming signals from standoff jammers (SOJs) due to the relatively high sidelobes of the system track antenna. Since the geometry required to position a jammer in the track antenna main lobe has a low probability of occurrence, the highest probability is that the jammer will intercept the antenna pattern in its sidelobes. Therefore, antenna design techniques incorporated into an operational antenna system which reduces the antenna sidelobes can provide significant improvement in microwave vulnerability to jamming signals. However, these techniques for reducing antenna sidelobe levels must not compromise other performance characteristics of the antenna, for optimum operation of the tracking system. Currently there are two types of monopulse antennas commonly used in current active and semi-active radar systems. These types of monopulse antennas are the parabolic reflectors and slot arrays. Each type of antenna has inherent design limitations which result in sidelobe levels much higher than desired for use in tactical situations.
The reflector-type track antenna is one type of monopulse antenna that is usually comprised of a parabolic dish and a four-port waveguide monopulse feed system. This type of antenna realizes amplitude comparison monopulse radiation patterns. In a parabolic track antenna feed system, however, produces considerable aperture blockage. As a result of the blockage effects, it is typical that a large-scale parabolic track antenna will produce sidelobes that are only 16 to 22 dB below the main beam in either or both the elevation and azimuth planes.
The other type of commonly used monopulse antenna is the slotted array antenna. This antenna type is typically used as a track or seeker antenna for radar systems because of its low profile. The conventional slotted array antenna uses phase comparison monopulse processing in its operation. Planar slotted array antennas typically exhibit lower sum sidelobes, over a limited bandwidth, than reflector-type antennas. However, slotted array antennas produce high vestigial lobes in both the azimuth and elevation difference channels, typically 11 to 15 dB below the main beam. These high sidelobes are a result of the aperture distribution being configured to optimize the sum channel performance in terms of gain and beamwidth.
The slotted array antenna uses phase comparison monopulse processing in which difference patterns are obtained by comparing phase values between corresponding halves of the array. For this reason, the difference-mode amplitude distribution suffers a severe discontinuity at the array center. This amplitude distribution discontinuity is also a factor in the high sidelobe levels common to this type of antenna.
In the standoff jamming environment typical of most combat scenarios, track antenna systems need to have sidelobe levels on the order of -30 dB to achieve adequate performance levels. The typical monopulse antenna, either reflector-type or slotted array-type, suffers substantial degradation in performance in the jamming environment. Furthermore, the typical slotted array antenna is perhaps the more vulnerable of the two types of track antennas. The most detrimental effect on system performance is most evident in a phase-comparison monopulse system when jammer noise is received through one of the difference channels.
It is, therefore, an object of the present invention to provide a novel and improved low-sidelobe slotted array amplitude-comparison monopulse track antenna utilizing traveling wave techniques.